To the avid recreational vehicle driver, having available a smaller vehicle to use after reaching a campsite proves to be useful. For example, it is far easier to park a smaller vehicle in the parking lot outside of a grocery store than a full-sized recreational vehicle. The towing of a smaller vehicle has become a part of the recreational vehicle such that the towed vehicle is regularly referred to as a “dinghy” analogizing the operation of a recreational vehicle to piloting a boat. A dinghy eliminates the need to break camp and stow everything each time the driver needs (or wants) to venture away from the campground. Additionally, the dinghy can stow gear securely when recreational vehicle storage is filled (within weight restrictions), and there is the sense of security engendered by having a spare vehicle for immediate transportation in the event of an emergency, such as an injury requiring attention at a local emergency room.
Nonetheless, despite the acknowledged utility of towing a towed vehicle or “dinghy”, there are also acknowledged difficulties. One arrangement to facilitate towing is use of the simple tow bar where all four of the towed vehicle's wheels will rotate in contact with the surface of the roadway (as opposed to using a dolly or trailer to support two or four of the wheels respectively). Among recreational vehicle owners, this arrangement is known as flat towing, or four-wheels-down towing and employs a tow bar between the towed and towing vehicles. Importantly, in using a tow bar to pull the towed vehicle, the recreational vehicle must then both accelerate and decelerate a mass that is greater by approximately one sixth of the recreational vehicle's own mass. While beefing up the engine in the recreational vehicle can usually appropriately address the acceleration of the combined mass of the two vehicles, stopping that combined mass presents another issue.
Even if the braking capacity of the recreational vehicle is sufficient to slow the combined mass of the recreational vehicle and the towed vehicle, when the only braking is that applied by the towing vehicle, the stopping distance from any given speed will be lengthened significantly. The momentum the towed vehicle contributes much more mass that must also be slowed along with the towing vehicle. When compared to stopping without the towed vehicle, decelerating that additional mass simply requires more braking force to slow the vehicles.
A solution readily asserts itself: each of the towing and towed vehicles have braking systems sufficient to safely stop those vehicles when used as separate vehicles. If both vehicles could suitably brake its own mass with its own braking system, the composite train of towing and towed vehicles can be slowed efficiently. Many recreational vehicle drivers elect to implement the towed vehicle's own braking mechanism by means of remote activation to enhance the braking action of the vehicles when used together. Generally termed “supplemental braking mechanisms,” these systems include instrumentation used to activate the towed vehicle's braking system in concert with that of the towing vehicle. In applying the supplemental braking, the total necessary force to decelerate the combined mass of the recreational vehicle and the towed vehicle is spread across the eight or more wheels the two vehicles comprise, thereby allowing each wheel to slow the vehicles rather than merely the four or so the towing vehicle controls.
In rough cut, this solution should work but the problem is in applying the brakes appropriately in the towed vehicle. Nonetheless, the need for stopping each of the towed and towing vehicles is universally recognized. For example, in executing panicked stops, the need for supplemental braking is clearly demonstrated. Without supplemental braking, stopping distances are simply too long to avoid collision or catastrophe, and such stops are the leading cause of towing vehicle braking system failure. So recognized is this danger that nearly every state and Canadian province requires a supplemental braking system on all towed vehicles over a certain weight, in the same way that they require brakes on trailers of a certain weight.
The tort system too recognizes the efficacy of supplemental braking. Because of the increased stopping distances when towing a vehicle without supplemental braking, accidents that could have been avoided still occur. In civil actions for negligence, if an accident that could have been avoided within the bounds of the towing vehicle's regular stopping distance occurs due to the added mass of the towed vehicle, a driver is much more likely to be found liable. The lack of a supplemental braking system is, as in failing to maintain a braking system in proper working order, a distinct act of negligence resulting in liability due to causation.
Several types of supplemental braking systems exist within the prior art. The most basic type of supplemental braking system is a portable, electric brake controller that applies the towed vehicle brakes at a fixed pressure over a duration defined by a voltage applied to the towing vehicle brake lights as received from the brake light switch in the towing vehicle. These systems act using relays to trigger brake “on” in the towed vehicle. The towed vehicle is either in a braked or freewheeling state based upon the voltage at the brake light. Many of the prior art systems are based can only apply a single selected pressure on the towed vehicle's brakes. This two-state system lacks the ability to exert a pressure proportionate to that applied in the recreational vehicle.
Just as with the brake lights that trigger it, the system is either “on” or “off” with no degrees of application. For that reason, the application of brakes in the towed vehicle is very uneven and only roughly matched to that in the towing vehicle. More sophisticated versions of the systems have an ability to predesignate a selected amount of pressure to apply to the towed vehicle braking system in response to the signal from the towing vehicle. Rather than simply applying a maximum pressure to the brake system, these systems apply pressure at a fixed pressure predesignated as, for example, “light”, “medium” or “heavy”. Trial and error allow a technician to “tune” the towed and towing vehicle train to select what proves, in practice, to be the smoothest of the designations available for this setting. Even when “tuned” in this fashion, the application of the brakes in the towed vehicle are not always optimal. For example, in light braking of the towing vehicle, the dinghy applies the preset pressure and acts as an anchor; in heavy braking, by contrast, the preset pressure might not supply enough braking force and the dinghy pushes the towing vehicle. Either option tends to lengthen stopping distances from what is optimal. Nonetheless, the stopping distances are far shorter than those produced by a system without supplemental braking. For that reason and the lack of expense, simplicity and reliability of these systems make them popular among RV enthusiasts.
A second system which seeks to achieve a closer to optimal braking distance is known as a proportionate braking system. Rather than a single pre-selected braking force, the proportionate system works by applying a braking force in proportion to deceleration experienced by the towed vehicle. Most proportionate systems exploit an inertial sensor such as a pendulum or accelerometer (such as a MEMS sensor implemented in an integrated circuit chip) to select a braking force in an intensity proportionate to the deceleration measured from a mounting point in the towing vehicle.
Because in this second system, the degree of braking is based upon sensed deceleration experience in the towing vehicle, in the case of hard deceleration, picking a proper relationship between sensed deceleration and applied braking in the towed vehicle dictates how closely the system approaches optimal braking. The ratio between sensed deceleration and degree of braking force applied in the towed vehicle is expressed as a coefficient. Selection of a suitable coefficient is necessary for better braking. For example, should the proportion of applied braking force relative to experienced deceleration be set to apply too great a pressure, the braking of the towed vehicle will drag the towing vehicle causing it to experience a further deceleration or, at the extreme, to simply lock up the brakes of the towed vehicle causing it to skid and drive the towing vehicle forward. At very least, this has the highly undesirable effect of wearing the towed vehicle brakes unduly while not shortening stopping distances. When properly adjusted, however, the system can approximate braking ranging from heavy-duty emergency braking, to general everyday braking and, in the extreme, slow-to-an-idle braking. Unfortunately, because the system is based upon the experienced and measured deceleration, there is a latency in the system, i.e. a delay between the onset of braking in the towing vehicle and the application of brakes in the towed vehicle, the vehicles do not brake in a synchronized manner. The differences in brake application tend to exert larger than necessary forces at the connection between the two vehicles and unevenly wear the brakes as between the two vehicles.
A third class of supplemental braking systems is known as “direct” because they use the towing system braking fluid to move a piston in a cylinder which, in turn depresses the towed vehicle brake pedal, just as that fluid is also used to close a brake caliper. Direct systems require a much more comprehensive installation process than most other systems, but they deliver superior braking. Like basic proportional systems, direct systems offer a whole spectrum of brake application intensities from emergency braking to slow-to-an-idle braking action; yet they have a far better response time and require little or no manual adjustment. Direct systems tap into the towing vehicle's brake lines using fluid (either air or hydraulic fluid) to move a piston in an actuator which operates similarly to the calipers on a brake. The actuator, in turn, applies a force on the fluid of the towed vehicle's brake system to sense the pedal movement in the towing vehicle so that the actuator in the dinghy can replicate that same timing and pressure in the towed vehicle. Essentially, a direct system acts as an additional circuit in the towing vehicle, such that brake pressure in either an air or hydraulic system in the towing vehicle drives a piston in a cylinder to similarly assert a pressure in the towed vehicle's system as would a foot on the brake pedal.
As stated above, because such a system functions as an extension of the brake system of the towing vehicle, installation and removal of the system tends to be a very elaborate task. Also, because the entire system must apply both vehicle's brakes, the necessary pedal travel in the towing vehicle lengthen and braking response tends to be “slushy.” Further, the actuators that link the systems must be very specifically engineered for the two vehicles such that pedal travel asserted by the actuator on the towed vehicle's brake pedal achieves the same slowing effect on the towed vehicle as the driver's foot asserts in the towing vehicle. These units tend to be very expensive because of the nearly custom nature of the installation.
What is needed in the art is a system that does not require elaborate installation and can be readily switched from a towed configuration to an operating configuration for ready use of the towed vehicle as a dinghy. Inherent or nontechnical synchronization of the asserted braking force to that experienced in the towing vehicle is necessary for safe operation.